Process of supplying water of controlled salinity

ABSTRACT

Process for producing an injection water for an oil reservoir by pressurizing water having a total dissolved solids content of 20,000 to 45,000 ppm and a sulfate concentration of 1,000 to 4,000 ppm to 350 to 1250 psi absolute; and dividing the water into first and second feeds for reverse osmosis (RO) and nanofiltration (NF) units respectively. The units are operated in single-pass, single-stage mode or single-pass, two-stage mode, the NF permeate has a pressure at least 5 psi higher than the RO permeate, and the recoveries of the RO and NF permeates are 35 to 75% and 35 to 60% by volume respectively. RO permeate and NF permeate are mixed in a ratio of 2:1 to 40:1 to provide injection water having a TDS content of 500 to 5,000 ppm, and a sulfate concentration of less than 7.5 ppm.

This application is the U.S. national phase of International ApplicationNo. PCT/GB2011/000032 filed 11 Jan. 2011 which designated the U.S. andclaims priority to European Application No. 10250063.4 filed 14 Jan.2010, the entire contents of each of which are hereby incorporated byreference.

The present invention relates to a process of providing a low salinityinjection water for an oil reservoir having a sufficient salinity toavoid formation damage and a sufficiently low sulfate anionconcentration to avoid souring of the reservoir, and to a desalinationsystem for producing such an injection water. In particular, the presentinvention provides a process and system for producing water ofcontrolled low salinity, controlled sulfate anion concentration andcontrolled multivalent cation concentration.

BACKGROUND OF THE INVENTION

As described in International patent application WO 2008/029124, it isknown to inject water of low salinity into an oil-bearing formation of areservoir in order to enhance the recovery of oil from the reservoir.

A problem associated with low salinity water-flooding is thatdesalination techniques may yield water having a salinity lower than theoptimal salinity for enhanced oil recovery. Indeed, the desalinatedwater may be damaging to the oil-bearing rock formation of the reservoirand inhibit oil recovery, for example, by causing swelling of clays inthe formation. There is an optimal salinity for the injection water thatprovides the benefit of enhanced oil recovery whilst avoiding formationdamage, and, the optimum value will vary from formation to formation.Typically, where an oil-bearing formation comprises rock that containshigh levels of swelling clays, formation damage may be avoided when theinjection water has a total dissolved solids content (TDS) in the rangeof 500 to 5,000 ppm, preferably, 1,000 to 5,000 ppm.

However, it is not desirable to mix a desalinated water of lowmultivalent cation content with a high salinity water such as seawaterowing to the high sulfate anion content and/or high multivalent cationcontent of the high salinity water. Thus, the high sulfate anion contentof the such mixed water streams may result in reservoir souring and/orthe precipitation of unacceptable levels of insoluble mineral salts(scale formation) when the injected water contacts precipitate precursorcations such as barium, strontium and calcium cations that are commonlypresent in the connate water of the formation. In addition, mixing ofdesalinated water with a high salinity water such as seawater may resultin the mixed water stream containing unacceptable levels of multivalentcations, in particular, calcium and magnesium cations. Thus, in order toachieve incremental oil recovery with a low salinity injection water,the ratio of the concentration of multivalent cations in the lowsalinity injection water to the concentration of multivalent cations inthe connate water of the reservoir should be less than 1, preferably,less than 0.9, more preferably, less than 0.8, in particular, less than0.6, for example, less than 0.5.

As described in International patent application WO 2007/138327, one wayin which the salinity of a water supply of overly low salinity might beincreased is by blending with water of higher salinity. According to WO2007/138327, this may be achieved by the steps of:

substantially desalinating a first feed supply of water to provide afirst supply of treated water of low salinity;

treating a second feed supply of water to provide a second supply oftreated water having a reduced concentration of divalent ions incomparison to the second feed supply and a higher salinity than thefirst supply of treated water; and

mixing the first supply of treated water and the second supply oftreated water to provide a supply of mixed water having a desiredsalinity suitable for injection into an oil bearing reservoir.

In preferred embodiments of the invention of WO 2007/138327, the firstfeed supply is substantially desalinated by a reverse osmosis processwhile the step of treating the second feed supply of water is preferablyperformed by nanofiltration.

Nanofiltration is commonly used in the oil industry to remove sulfateions from a source water. The treated water can then be injected into aformation without the risk of forming unacceptable levels of insolublemineral salts when the injected water contacts precipitate precursorcations present in the connate water of the formation. The invention ofWO 2007/138327 therefore permits the supply of a mixed water having thedesired salinity suitable for injection into the oil bearing reservoirand having a reduced level of sulfate anions thereby mitigating the riskof mineral scale precipitation either within the formation or inproduction wells.

It is known that injection of a water that contains high levels ofsulfate anions can stimulate the growth of sulfate reducing bacteriathat produce hydrogen sulfide as a metabolite resulting in souring of areservoir. Where it is desired to mitigate the risk of mineral scaleformation, the level of sulfate anions in the supply of mixed watershould be less than 40 ppm. However, where it is desired to mitigate therisk of souring in a reservoir, the level of sulfate anions in thesupply of mixed water should be as low as possible, for example, lessthan 7.5 ppm, preferably, less than 5 ppm.

SUMMARY OF THE INVENTION

It has now been found that it is necessary to carefully control theoperating conditions of the process of WO 2007/138327 in order toachieve a supply of mixed water of the desired total dissolved solidscontent for controlling formation damage and of the desired low sulfateanion concentration for controlling reservoir souring.

The present invention is therefore concerned with an improved processand plant for providing a mixed water stream of controlled salinity, andcontrolled low sulfate anion content for use as injection water for alow salinity waterflood whilst mitigating the risk of formation damage,and controlling souring in the reservoir.

Thus, according to a first embodiment of the present invention there isprovided a process of producing an injection water stream of controlledsalinity and controlled sulfate anion concentration that is suitable forinjection into an oil bearing formation of an oil reservoir, the processcomprising the steps of:

feeding a source water having a total dissolved solids content in therange of 20,000 to 45,000 ppm and a sulfate anion concentration in therange of 1,000 to 4,000 ppm, preferably, 1,500 ppm to 4,000 ppm to adesalination plant that comprises a plurality of reverse osmosis (RO)membrane units and a plurality of nanofiltration (NF) membrane unitswherein the source water is pressurised to a pressure in the range of350 to 1250 psi absolute, and dividing the source water to provide afeed water for the RO membrane units (hereinafter “RO feed water”) and afeed water for the NF membrane units (hereinafter “NF feed water”);if necessary, increasing the pressure of the RO feed water to a value inthe range of 900 to 1250 psi absolute before introducing the RO feedwater to the RO membrane units and withdrawing an RO permeate and an ROretentate from the RO membrane units wherein the RO membrane units areoperated in either a single-pass, single-stage mode or in a single-pass,two-stage mode and wherein the recovery of RO permeate is in the rangeof 35 to 75% by volume, preferably, 35 to 60% by volume based on thevolume of the RO feed water that is fed to the RO membrane units suchthat the RO permeate has a total dissolved solids contents of less than250 ppm, and a sulfate anion concentration of less than 3 ppm; ifnecessary, reducing the pressure of the NF feed water to a value in therange of 350 to 450 psi absolute before introducing the NF feed water tothe NF membrane units and withdrawing an NF permeate and an NF retentatefrom the NF membrane units wherein the NF membrane units are operated ina single-pass, single-stage mode and wherein the NF membrane units areoperated with a recovery of NF permeate in the range of 35 to 60% byvolume based on the volume of the NF feed water that is fed to the NFmembrane units such that the NF permeate has a total dissolved solidscontent in the range of 15,000 to 40,000 ppm, preferably, 15,000 to35,000 ppm, and a sulfate anion concentration of less than 40 ppm,preferably less than 30 ppm; andmixing at least a portion of the RO permeate and at least a portion ofthe NF permeate in a ratio in the range of 2:1 to 40:1, preferably, 4:1to 27:1, in particular, 10:1 to 25:1 to provide an injection waterhaving a total dissolved solids content in the range of 500 to 5,000ppm, preferably, 1,000 to 5,000 ppm, and a sulfate anion concentrationof less than 7.5 ppm, preferably, less than 5 ppm, more preferably lessthan 3 ppm.

The source water may seawater, estuarine water, a produced water, anaquifer water, or a waste water.

Preferably, the total dissolved solids content (TDS) of the RO permeateis in the range of 50 to 225 ppm, more preferably, 100 to 225 ppm, mostpreferably, 125 to 200 ppm, in particular, 150 to 175 ppm.

Preferably, the sulfate anion concentration of the RO permeate is in therange of 0.5 to 2.5 ppm, in particular, 0.5 to 1.5 ppm.

Preferably, the TDS of the NF permeate is not more than 15,000 ppm less,preferably not more than 10,000 ppm less than the TDS of the sourcewater.

Preferably, the sulfate anion concentration of the NF permeate is in therange of 10 to 28 ppm, more preferably 10 to 25 ppm, in particular, 15to 20 ppm.

The sulfate anion concentration of the injection water will be dependentupon the desired total dissolved solids content (TDS) for this streamand hence the mixing ratio for the RO permeate and NF permeate. Thus,the sulfate anion concentration of the injection water will increasewith increasing amounts of NF permeate in the mixed stream. Typically,the sulfate anion concentration for an injection water stream having atotal dissolved solids content of 1000 ppm is in the range of 1 to 2ppm, and the values for the range for the sulfate anion concentrationshould be scaled for injection waters of higher TDS.

An advantage of the process of the present invention is that in additionto providing an injection water having a sufficiently high TDS tomitigate the risk of formation damage and having a sufficiently lowsulfate concentration to mitigate the risk of souring in the reservoir,depending upon the choice of the source water, the injection water mayalso have a sufficiently low multivalent cation concentration for use asa low salinity injection water thereby achieving incremental oilrecovery from the reservoir.

Accordingly, the present invention is also concerned with an improvedprocess and plant for providing a mixed water stream of controlledsalinity, controlled low sulfate anion concentration and controlledmultivalent cation concentration for use as injection water for a lowsalinity waterflood whilst mitigating the risk of formation damage, andcontrolling souring in the reservoir.

Thus, in a second embodiment of the present invention, there is provideda process of producing an injection water stream of controlled salinity,controlled sulfate anion concentration and controlled multivalent cationconcentration that is suitable for injection into an oil bearingformation of an oil reservoir, the process comprising the steps of:feeding a source water having a total dissolved solids content in therange of 20,000 to 45,000 ppm, a sulfate concentration in the range of1,000 to 4,000 ppm, preferably, 1,500 ppm to 4,000 ppm, and amultivalent cation concentration in the range of 700 to 3,000 ppm,preferably 1,000 to 3,000 ppm, more preferably, 1,500 to 2,500 ppm to adesalination plant that comprises a plurality of reverse osmosis (RO)membrane units and a plurality of nanofiltration (NF) membrane unitswherein the source water is pressurised to a value in the range of 350to 1250 psi absolute, and dividing the source water to provide an ROfeed water and an NF feed water;

if necessary, increasing the pressure of the RO feed water to a value inthe range of 900 to 1250 psi absolute before introducing the RO feedwater to the RO membrane units and withdrawing an RO permeate and an ROretentate from the RO membrane units wherein the RO membrane units areoperated in either a single-pass, single-stage mode or in a single-pass,two-stage mode and wherein the recovery of RO permeate is in the rangeof 35 to 75% by volume, preferably, 35 to 65% by volume based on thevolume of the RO feed water that is fed to the RO membrane units suchthat the RO permeate has a total dissolved solids contents of less than250 ppm, a sulfate anion concentration of less than 3 ppm, and amultivalent cation content of up to 10 ppm;if necessary, reducing the pressure of the NF feed water to a value inthe range of 350 to 450 psi absolute before introducing the NF feedwater to the NF membrane units and withdrawing an NF permeate and an NFretentate from the NF membrane units wherein the NF membrane units areoperated in a single-pass, single-stage mode with a recovery of NFpermeate in the range of 35 to 60% by volume based on the volume of theNF feed water that is fed to the NF membrane units such that the NFpermeate has a total dissolved solids content in the range of 15,000 to40,000 ppm, preferably, 15,000 to 35,000 ppm, a sulfate anionconcentration of less than 40 ppm, preferably, less than 30 ppm and amultivalent cation content of up to 200 ppm, preferably up to 150 ppm,more preferably up to 100 ppm; and

-   -   mixing at least a portion of the RO permeate and at least a        portion of the NF permeate in a ratio in the range of 2:1 to        40:1, preferably, 4:1 to 27:1, in particular, 10:1 to 25:1 to        provide an injection water having a total dissolved solids        content in the range of 500 to 5,000 ppm, preferably, 1,000 to        5,000 ppm, a sulfate anion concentration of less than 7.5 ppm,        preferably, less than 5 ppm, more preferably less than 3 ppm and        a multivalent cation content of up to 50 ppm.

Again, the source water may seawater, estuarine water, a produced water,an aquifer water, or a waste water.

The preferred TDS for the source water, the RO permeate, the NF permeateand the injection water are as given above for the first embodiment ofthe present invention.

The source water preferably has a calcium cation concentration in therange of 200 to 600 ppm. Preferably, the source water has a magnesiumcation concentration in the range of 500 to 2000 ppm.

The preferred concentrations of sulfate anions in the RO permeate, NFpermeate and injection water are as given above for the first embodimentof the present invention.

Preferably, the concentration of multivalent cations in the RO permeateis in the range of 1 to 10 ppm, preferably, 1 to 5 ppm, in particular, 1to 3 ppm.

Preferably, the concentration of multivalent cations in the NF permeateis in the range of 50 to 200 ppm, preferably, 50 to 150 ppm.

The concentration of multivalent cations in the injection water will bedependent upon the desired TDS for this stream and hence the mixingratio for the RO permeate and NF permeate. Thus, the multivalent cationconcentration of the injection water will increase with increasingamounts of NF permeate in the mixed stream. Typically, the multivalentcation concentration for an injection water stream having a totaldissolved solids content of 1000 ppm is in the range of 2 to 10 ppm, andthe values for the range of multivalent cation concentration should bescaled for injection waters of higher TDS.

As discussed above, where it is desired to achieve incremental oilrecovery with a low salinity injection water, the ratio of themultivalent cation concentration of the low salinity injection water tothe multivalent cation concentration of the connate water should be lessthan 1. The multivalent cation concentration of a connate water istypically several times greater than the multivalent cationconcentration of the injection water formed by mixing the RO permeateand the NF permeate according to the process of the present invention.Accordingly, the injection water has the desired low salinity anddesired low multivalent cation concentration to achieve incremental oilrecovery when injected into a hydrocarbon-bearing formation of areservoir whilst having a sufficient content of total dissolved solidsto prevent formation damage and a sufficiently low sulfate concentrationto mitigate the risk of souring in the reservoir (as well as mitigatingthe risk of precipitation of insoluble mineral salts in the formationand/or production wells).

Typically, the formation into which the injection water of controlledsalinity (controlled TDS), controlled low sulfate anion concentrationand controlled low multivalent cation concentration is injected is anoil-bearing sandstone formation that contains a high content of swellingclays, for example, smectite clays. By high content of swelling clays ismeant a content of swelling clays of 10% by weight or greater, forexample, a content of swelling clays in the range of 10 to 30% byweight.

Typically, in these first and second embodiments of the presentinvention, the RO permeate and the NF permeate are mixed in a volumeratio (volume of RO permeate to volume of NF permeate) of 2:1 to 40:1,in particular, 4:1 to 27:1, in particular, 10:1 to 25:1. The personskilled in the art will understand that the particular mixing ratio willdepend on a one or more of the following factors:

(a) the salinity of the source water;

(b) the sulfate concentration of the source water;

(c) the multivalent cation concentration of the source water;

(d) the temperature at which the RO and NF membrane units are operated;

(e) the percentage volume recovery at which the RO and NF membrane unitsare operated;

(f) the desired salinity of the injection water;

(g) the desired sulfate anion concentration of the injection water; and

(h) the desired multivalent cation concentration of the injection water.

Factors (f), (g) and (h) are, in turn, dependent on characteristics ofthe reservoir into which it is desired to inject the treated water suchas the amount of swelling clays, the levels of sulfate reducingbacteria, and the multivalent cation concentration of the connate water.Thus, depending on the mixing ratio of the RO permeate to the NFpermeate, the injection water stream will have a salinity sufficient tocontrol formation damage, a sufficiently low sulfate concentration tocontrol souring in the oil reservoir, and a sufficiently low multivalentcation concentration that the ratio of the multivalent cationconcentration of the injection water to that of the connate water of theformation is less than 1.

Advantageously, the ratio of mixing of the RO permeate and the NFpermeate is controlled in accordance with a measured variable. Thecontrol may be automatic and a feed-back control system may be employed.

The measured variable may be a property of the injection water, forexample, the measured variable may relate to the salinity (TDS content)of the injection water, and preferably is the conductivity of theinjection water. The conductivity is a measure of the TDS content of theinjection water. Alternatively, or additionally, the measured variablemay relate to the concentration of multivalent anions in the injectionwater or in the NF permeate, or the concentration of selected divalentanions, such as sulfate anions, in the injection water or in the NFpermeate. Alternatively, or additionally, the measured variable mayrelate to the concentration of multivalent cations in the injectionwater or in the NF permeate, or the concentration of selectedmultivalent cations, such as calcium cations and/or magnesium cations inthe injection water or in the NF permeate.

The flow rate of the injection water stream or of the source waterstream may also be controlled in accordance with a measured variable.

By “single-pass, single-stage” mode is meant that the feed water ispassed through a plurality of individual membrane units that arearranged in parallel. Thus, a feed water is passed to each of themembrane units and a permeate stream and a retentate stream is removedfrom each of the membrane units. The permeate streams are then combinedto form a combined permeate stream. The percentage recovery of themembrane units when operated in “single-pass, single stage” mode is:[(volume of the combined permeate stream/the volume of thefeedwater)×100]. These volumes are determined over a set time period,for example, volume of feed water processed in one day and volume ofcombined permeate stream produced in one day.

By “single-pass, two stage” mode is meant that that the feed water isfed to the first of two membrane units that are arranged in series withthe retentate from the first membrane unit being used as feed water tothe second membrane unit in the series. Typically, there may be aplurality of first membrane units that are arranged in parallel and aplurality of second membrane units arranged in parallel. Generally,there will be fewer second membrane units than first membrane units asthe second membrane units will process a smaller volume of water over aset time period than the first membrane units. Typically, the permeatestreams from the first membrane units are mixed to give a first permeatestream and the retentate streams from the first membrane units are mixedto form a first retentate stream. The first retentate stream is thenused as feed water to the plurality of second membrane units that arearranged in parallel. The permeate streams from the second membraneunits are then typically mixed to give a second permeate stream. Thesecond permeate stream is then combined with the first permeate streamto give a combined permeate stream. The retentate streams from thesecond membrane units are typically mixed to give a combined retentatestream that is discharged from the desalination plant. However, thereare other ways of combining the various streams when operating aplurality of membrane units in a “single-pass, two stage” mode that arewithin the common general knowledge of the person skilled in the art.

The percentage recovery of the membrane units when operated in “singlepass, two stage” mode is: [(volume of the first permeate stream from thefirst membrane units+volume of the second permeate stream from thesecond membrane units)/the volume of the feedwater to the first membraneunits))×100]. These volumes are determined over a set time period of,for example, one day.

The NF membrane units are preferably operated in “single-pass,single-stage” mode. The RO membrane units are preferably operated ineither “single-pass, single-stage” mode or single pass, two stage” mode,in particular, “single-pass, single-stage” mode.

In the present invention, the RO membrane units are operated with apressure differential across the membrane that provides a recovery of ROpermeate in the range of 35 to 75% by volume, preferably, 35 to 65% byvolume, more preferably, 35 to 60% by volume, most preferably 45 to 55%by volume, in particular, 50 to 55% by volume, based on the volume ofthe RO feed water.

Typically, the pressure differential across the RO membrane units(pressure of the RO feed water-pressure of the combined RO retentate) isin the range of 25 to 100 psi, preferably 35 to 75 psi, for example,about 50 psi. Accordingly, the retentate streams that exit the ROmembrane units are at a relatively high pressure. Preferably, some orall of the RO retentate streams that are to be discharged from the ROmembrane units may be combined and the resulting combined RO retentatestream is passed through a hydraulic recovery unit, for example, ahydraulic recovery turbine or a turbocharger that is coupled to abooster pump for the RO feed water. Thus, the hydraulic recovery unitrecovers energy from the RO retentate unit and uses this recoveredenergy to boost the pressure of the RO feed water thereby reducing thepower requirements for the desalination plant. Typically, the pressureof the combined RO retentate stream downstream of the hydraulic recoveryunit is less than 100 psig, preferably, in the range of 10 to 75 psig,in particular, 20 to 55 psig, for example, 10 to 50 psig.

In the present invention, the NF membrane units are operated with apressure differential across the membrane that provides a recovery of NFpermeate in the range of 35 to 60% by volume, preferably 45 to 55% byvolume, in particular, about 50% by volume, based on the volume of theNF feed water.

Typically, the pressure differential across the NF membrane units(pressure of the NF feed water-pressure of the NF retentate) is in therange of 25 to 100 psi. Accordingly, the pressure of the combined NFretentate stream is typically too low to warrant recovering energy fromthis stream. However, if desired energy may also be recovered from theNF retentate stream using a hydraulic recovery unit.

Preferably, the desalination plant comprises at least two membranetrains, preferably, 2 to 12, more preferably, 2 to 8, for example, 2 to6, in particular, 4 to 6 membrane trains, wherein each train comprises aplurality of RO membrane units and a plurality of NF membrane units.Typically, the ratio of RO membrane units to NF membrane units in eachtrain is in the range of 2:1 to 40:1, preferably, 4:1 to 27:1, inparticular 10:1 to 25:1. Accordingly, an advantage of the desalinationplant of present invention is that a separate NF train is eliminatedwhich reduces space and weight considerations, which is of particularconcern for offshore facilities where the plant is located on a platformor a Floating Production Storage and Offloading (FPSO) facility. Inaddition, the incorporation of NF units in each train of thedesalination plant of the present invention means that an injectionwater of the desired composition remains available even if one or moreof the trains of the desalination plant is out of action for cleaning,maintenance, or the event of an emergency.

Each train may be provided with dedicated pumping systems and,optionally, dedicated hydraulic recovery systems. Alternatively, theremay be a common pumping system and, optionally, a common hydraulicrecovery system, for the plurality of trains.

Preferably, the membrane units of each train are arranged in a pluralityof rows or racks. In order to reduce the footprint of the desalinationplant, it is preferred that these rows are arranged one above another.Preferably, each membrane train comprises between 3 to 15 rows,preferably, between 6 to 12 rows. Generally, there are between 4 and 16membrane units, preferably, between 6 to 12 membrane units in each row.Typically, the NF membrane units are arranged together, for example, allor a portion of the membrane units of one or more of the rows may be NFmembrane units. Where the RO membrane units are operated in“single-pass, two-stage” mode, it is preferred that the first membraneunits in the series are arranged together in one or more rows and thesecond membrane units in the series are also arranged together in one ormore rows.

Preferably, the membranes of the NF and RO membrane units are spiralwound membranes. Spiral wound membranes typically have a length in rangeof 40 to 60 inches (1.08 to 1.52 meters) and an external diameter in therange of 2.5 to 18 inches (6.36 to 45.7 cm).

The NF membrane units and the RO membrane units of each train comprise aplurality of pressure containment housings that contain at least onemembrane, preferably, 4 to 8 membranes. The housings may be formed fromglass reinforced resin or from steel. Typically, each housing canwithstand a pressure in excess of 1100 psi absolute, preferably inexcess of 1300 psi absolute, in particular, in excess of 1400 psiabsolute. Typically, the housings are cylindrical in shape and arearranged parallel to one another, in rows (or racks), with thelongitudinal axes through the housings lying in a substantiallyhorizontal plane.

In a first preferred aspect of the present invention, the source watermay be pressurised to the desired feed pressure for the RO membraneunits of each train, for example, using a high pressure pump. The sourcewater is then divided to provide the RO feed water for the RO membraneunits and the NF feed water for the NF membrane units. Where the ROmembrane units of the train are operated in single-pass, two stage mode,the desired feed pressure for the RO membrane units refers to thepressure at which the first membrane units of the series are operated.

Typically, for this first preferred aspect of the present invention,each membrane train is provided with a feed header for the RO feedwater, a feed header for the NF feed water, a retentate header for acombined retentate stream and a permeate header for a combined permeatestream. The RO feed header and NF feed header are in fluidiccommunication with a feed line for the source water. Where a rowcontains only RO membrane units or only NF membrane units, a common feedline is provided leading from the appropriate feed header (RO feedheader and NF feed header respectively) to the individual membrane unitsof each row. Similarly, a common retentate flow line and a commonpermeate flow line lead from the individual membrane units of each rowto the retentate and permeate headers respectively. Where a row containsboth RO and NF membrane units, a dedicated common feed line is providedfor the RO membrane units leading from the RO feed header and a furtherdedicated common feed line for the NF membrane units leading from the NFfeed header. In a similar manner, the RO and NF membrane units of therow may be provided with dedicated common retentate lines and dedicatedcommon permeate flow lines.

A flow controller may be provided in the or each common NF feed line soas to control the split of the source water between the RO membraneunits and the NF membrane units. As discussed above, the inlet or feedpressure for the NF units is in the range of 350 to 450 psi absolute, inparticular, 380 to 420 psi absolute, for example, about 400 psiabsolute. Where the pressure of the source water is above the desiredinlet pressure for the NF membrane units, a pressure let down valve maybe provided in the or each common NF feed line such that the pressuremay be reduced to the desired inlet pressure. Alternatively, a controlvalve may be provided in the or each feed line for the NF membrane unitswherein the control valve regulates the flow of the source water to theNF membrane units and also lets down the pressure of the source water tothe desired inlet pressure for the NF membrane units. It is alsoenvisaged that a flow controller may be provided upstream of the NF feedheader thereby controlling the split of the source water between the ROfeed header and the NF feed header and hence the split of the sourcewater between the RO membrane units and NF membrane units. If necessary,a pressure let-down valve may also be provided upstream of the NF feedheader. Alternatively, a control valve of the type described above maybe provided upstream of the NF feed header.

In second preferred aspect of the present invention, the source watermay be at a pressure below the desired inlet pressure for the ROmembrane units. It is therefore necessary to boost the pressure of theRO feed water using a booster pump. Preferably, the booster pump iscoupled to a hydraulic recovery system that recovers energy from thecombined retentate stream that exits the RO membrane units. Thishydraulic recovery system may be a hydraulic turbine. Thus, a shaft ofthe turbine may drive a shaft of the booster pump. These shafts may beconnected via a gear system. However, the person skilled in the art willunderstand that additional energy must be supplied to the booster pumpif the RO feed water is to reach the desired inlet pressure for the ROmembrane units.

Typically the source water is pressurised to a value in the range of 350to 1100 psi absolute prior to being divided to provide the RO feed waterand NF feed water. It is preferred to pressurise the source water to avalue above the inlet pressure for the NF membrane units prior todividing the source water to give the RO and NF feed waters. Thus, it ispreferred that the pressure of the source water is in the range of 600to 1100 psi absolute, preferably 700 to 900 psi absolute.

In this second preferred aspect of the present invention, each membranetrain is provided with a first feed header for the RO feed water (whichhas been boosted in pressure to a pressure in the range of 900 to 1250psi absolute), a second feed header for the NF feed water (which hastypically been let down in pressure to a pressure in the range of 350 to450 psi absolute), a retentate header for a combined retentate streamand a permeate header for a combined permeate stream. Where a rowcontains only RO membrane units, a common feed line is provided leadingfrom the RO feed header to the individual RO membrane units of each row.Similarly, a common retentate flow line and a common permeate flow lineleads from the individual RO membrane units of each row to the retentateand permeate headers respectively. Where a row contains NF membraneunits, a common feed line is provided for the NF membrane units leadingfrom the NF feed header. Similarly, the NF membrane units of the row areprovided with common retentate and common permeate flow lines that leadto the retentate and permeate headers respectively. Where a row containsboth NF membrane units and RO membrane units, a dedicated common RO feedline, a dedicated common RO retentate line and a dedicated common ROpermeate line are provided for the RO membrane units. Similarly, adedicated common NF feed line, a dedicated common NF retentate line anda dedicated common NF permeate line are provided for the NF membraneunits.

In a similar manner to the first preferred aspect of the presentinvention, a flow controller is typically provided so as to control thesplit of the source water between the RO feed header and the NF feedheader. Typically, a pressure let down valve is provided upstream of theNF feed header such that the pressure may be let down to the desiredinlet pressure for the NF membrane units. However, it is also envisagedthat a pressure let down valve may be provided in the or each common NFfeed line. Alternatively, as described above, a control valve may beprovided upstream of the NF feed header thereby controlling both thesplit of the source water and the pressure of the NF feed water.

The provision of NF membrane units in each train of the desalinationplant allows the plant to continue to operate and produce water of thedesired salinity, sulfate anion concentration and multivalent cationconcentration in the event that it becomes necessary to shut down one ormore of the trains for maintenance or cleaning.

Typically, the membrane(s) contained in each membrane unit in a row areprovided with water tight pressure fittings for connection to (i) thecommon feed line, (ii) the common permeate flow line and (iii) thecommon retentate feed line.

Suitably, a back pressure valve is provided on the or each common NFpermeate flow line upstream of the mixing point for the NF permeate andRO permeate. Alternatively, where there is more than one common NFpermeate line, these lines may lead to a combined NF permeate line andthe back pressure valve may be provided in this combined NF permeateline. The back-pressure valve ensures that the pressure of the NFpermeate is sufficiently above the pressure of the RO permeate to allowthe NF permeate to be injected into the permeate header. The resultingmixed permeate stream is the injection water stream that then enters aninjection water flow line. Suitably, the back-pressure valve opens whenthe pressure of the NF permeate exceeds a pre-set pressure and allowssufficient flow of NF permeate through the valve to maintain thepressure of the NF permeate at above the pre-set pressure. Typically,the pre-set pressure of the back-pressure valve is at least 5 psi higherthan the pressure of the RO permeate. Generally, the pressure of the ROpermeate, will be in the range of 10 to 75 psi absolute, preferably, 20to 55 psi absolute.

Preferably, the source water may have undergone at least one of:filtration to remove particulate matter, chlorine scavenging, dosingwith a biocide, deaeration, and dosing with a scale inhibitor. Thesetreatments may be performed on the first and/or NF feed waters but inorder to reduce space and weight of the plant, it is preferred toperform these treatments on the source water feed prior to dividing thesource water to form the RO feed water and NF feed water.

As an alternative to deaerating the source water upstream of thedesalination plant, it is envisaged that a deaerator may be provideddownstream of the desalination plant in order to control corrosion inthe injection lines, injection pumps and injection wells. An advantageof providing a downstream deaerator is that the volume of water that isdeaerated is substantially less than if the deaerator was arrangedupstream of the desalination plant. However, having a deaerator upstreamof the desalination plant reduces the risk of corrosion within thedesalination plant and therefore allows for the use of cheaper steels.It may therefore be advantageous to provide a deaerator upstream of thedesalination plant.

In a further embodiment of the present invention there is provided adesalination plant comprising a plurality of trains each comprising aplurality of RO membrane units and a plurality of NF membrane unitswherein the ratio of RO membrane units to NF membrane units in eachmembrane train is in the range of 2:1 to 40:1, preferably, 4:1 to 27:1,in particular 10:1 to 25:1, and wherein each membrane train is providedwith:

-   -   (a) a feed line for a source water wherein the feed line divides        to provide a feed line (or header) for the RO membrane units and        a feed line (or header) for the NF membrane units,    -   (b) a permeate line (or header) for the RO membrane units and a        permeate line (or header) for the NF membrane units wherein the        permeate lines combine to provide an injection water line;    -   (c) a retentate line (or header) for the RO membrane units and a        retentate line (or header) for the NF membrane units; and    -   (d) a flow controller and pressure let-down valve on the NF feed        line.

As discussed above, it is envisaged that flow controller and a pressurelet-down valve may be combined in the form of a control valve.

Preferably, the membrane units are arranged in rows placed one above theother. Preferably, the NF membrane units are arranged together in one ormore rows. Typically, each membrane train comprises between 3 and 15rows with each row comprising between 4 and 16 membrane units.

Preferably, a booster pump is provided on the RO feed line and ahydraulic recovery unit on the RO retentate line wherein the hydraulicrecovery unit is coupled to the booster pump. Typically, the hydraulicrecovery unit is a hydraulic turbine of the type described above.Alternatively, the hydraulic recovery unit may be a turbocharger.

The capacity of the desalination plant should be sufficient to meet thelow salinity injection water requirements for the oil reservoir.Typically, each train of the desalination plant is capable of producingbetween 20,000 and 200,000 bbls of water per day, for example, 40,000and 60,000 bbls of water per day of the desired low salinity and desiredlow sulfate anion concentration.

Preferably, a back-pressure valve is provided on the NF permeate line(s)so as to allow accurate metering of the NF permeate into the RO permeatethereby resulting in the production of an injection water having thedesired characteristics, e.g. desired controlled salinity, desiredsulfate anion concentration and desired multivalent cationconcentration.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to thefollowing Examples and Figures, in which:

FIG. 1 is a schematic diagram of the process and desalination plant ofthe present invention,

FIG. 2 is a schematic diagram of a modification of the process anddesalination plant of the present invention, and

FIG. 3 is a schematic diagram of a train of membrane units for use inthe process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, a feed of source water 1 for a desalination plant thatcomprises a plurality of RO membrane units (shown schematically at 2)and a plurality of NF membrane units (shown schematically at 3) issupplied to a pump 4 that increases the pressure of the source water 1to a desired value in the range of 900 to 1250 pounds per square inchabsolute (psi absolute). Preferably, the source water has been treatedupstream of the pump 4. Thus, the source water may be chlorinated,strained and passed through a filtration system to remove particulatematter down to a desired level, typically to a level commensurate with aSilt Density Index (SDI 15 minutes) of less than 5 and preferably lessthan 3. The SDI reduction can be achieved using a variety of wellunderstood methods including microfiltration, ultrafiltration, mediafilter systems and cartridge filtration. The filtrate may be dosed witha chlorine scavenger downstream of the filtration system, to remove anyresidual free chlorine that could otherwise damage the membranes of themembrane units that are disposed downstream of the pump 4. The sourcewater may also be passed through a deaerator to remove oxygen therebycontrolling corrosion in the desalination plant and downstream of thedesalination plant, for example, in injection lines, injection pumps,and injection wells. If desired, the source water may also be dosed witha biocide upstream of pump 4 in order to control biological activitywhich might otherwise occur in the system. Scale inhibitor may also bedosed into the source water upstream of the pump 4 in order to minimisescaling on the downstream membrane surfaces.

The source water 1 is divided downstream of pump 4 to provide an RO feedwater 5 for the plurality of RO membrane units 2 and an NF feed water 6for the plurality of NF membrane units 3. Preferably, these membraneunits are arranged in one or more membrane trains which are described inmore detail below by reference to FIG. 3. The split of the RO feed water5 and NF feed water 6 is controlled by flow controller 7. The pressureof the NF feed water 6 is then reduced to a value in the range of 350 to450 psi absolute via a pressure let-down valve 8 prior to being fed tothe NF membrane units 3. The RO retentate 9 removed from the RO membraneunits 2 and the NF retentate 10 removed from the NF membrane units 3 arerejected while the RO permeate 11 removed from the RO membrane units 2and the NF permeate 12 removed from the NF membrane units 3 are combinedto provide an injection water 13 of controlled salinity and controlledsulfate anion concentration.

FIG. 2 is a modification of the process and desalination plant of FIG. 1in which pump 4 increases the pressure of the source water to a value of700 psi absolute before dividing the source water into an RO feed water5 and an NF feed water 6. The RO feed water 5 for the plurality of ROmembrane units 2 is then boosted in pressure to the desired operatingpressure of the RO membrane units 2 (1100 psig) using a booster pump 14thereby generating a pressurised RO feed water 16. A hydraulic recoveryturbine 15 is coupled to the booster pump 15 and recovers energy fromthe retentate 9 that is removed from the RO membrane units 2 therebygenerating a reduced pressure retentate 17 that is rejected from thedesalination plant. The pressure of the NF feed water 6 is reduced to avalue in the range of 350 to 450 psi absolute via a pressure let-downvalve 8 as described in respect of FIG. 1.

FIG. 3 illustrates a transverse cross-section through a membrane train20 for use in the process and desalination plant of the presentinvention. The membrane train 20 comprises seven rows 21 with each rowcomprising eight membrane units 22 arranged in substantially horizontalplanes one above another. However, it is envisaged that a membrane trainmay comprise more than or less than seven rows and that each row maycomprise more than or less than eight membrane units. Each of themembrane units comprises a housing that is substantially cylindrical inshape having a length in the range of 35 to 345 inches (0.89 to 8.76meters), and an internal diameter in the range of 2.5 to 75 inches (6.35cm to 1.91 meters). The housing contains at least one spiral woundmembrane (not shown), preferably, two to four spiral wound membranes,preferably three or four spiral wound membranes. Each of the spiralwound membranes are wound in the form of a cylinder and have a length inrange of 30 to 60 inches (0.762 to 1.52 meters) and an external diameterin the range of 2.5 to 18 inches (6.36 to 45.7 cm). A typical membranehas a length of about 40 inches (1.02 meters) and a diameter of about 8inches (20.3 cm). Where a housing contains more than one membrane, themembranes are typically arranged end to end, in which case the housinggenerally has an internal diameter of up to 18 inches (45.7 cm) and alength of up to 345 inches (8.76 meters).

The membrane train 20 has a feed header 23 a for the RO source water anda feed header 23 b for the NF feed water. The RO feed header 23 a istypically arranged substantially vertically, at a mid-point of the trainsuch that half of the RO membrane units of each row are arranged oneither side thereof. For example, where each membrane row has eight ROmembrane units, four RO membrane units may be provided on either side ofthe RO feed header 23 a. The majority of the membrane units of the trainare reverse osmosis (RO) units and the remainder are nanofiltration (NF)membrane units with the ratio of RO units to NF units being dependent onthe desired mixing ratio of RO permeate and NF permeate which in turn isdependent on the % volume recovery of permeate from the RO and NFmembrane units. FIG. 3 shows four NF membrane units 24 arranged in thebottom row to the left of the NF feed header 23 b. However, it is alsoenvisaged that the NF feed header 23 b may be arranged at the mid-pointof the train.

A plurality of common feed lines lead from the feed headers to the rowsof the membrane train. Thus, the bottom row is provided with a firstcommon feed line 25 leading from the RO feed header 23 a to the four ROmembrane units arranged to the left of the RO feed header 23 a and asecond common feed line 26 leading from the NF feed header 23 b to thefour NF membrane units 24 arranged to the left of the NF feed header 23b. Thus, the water that flows through the second common feed line 26 isthe NF water feed. A flow control valve and pressure let-down valve (notshown) are provided in the second common feed line 26 for reducing thepressure of the NF water feed to the operating pressure of the NFmembrane units 24. The pressure let-down valve is controlled via apressure controller (not shown) such that the pressure of the NF feedwater downstream of the valve is in the range of 350 to 450 psiabsolute. The NF membrane units 24 are single-pass single-stage unitswith the retentate from the NF membrane units being rejected by a commonretentate reject line (not shown) that leads to an NF retentate header(not shown). The permeate from each NF membrane unit is fed to a commonNF permeate line (not shown) that leads to an NF permeate header (notshown).

The remaining rows of the train (the upper six rows) are each providedwith a common feed line 27 leading from the RO feed header 23 a to eachof the RO membrane units of the row. Thus, the water that flows throughthe first common feed line 25 of the bottom row and the common feedlines 27 of the upper six rows is the RO feed water for the RO membraneunits. Like the NF membrane units, the RO membrane units shown in FIG. 3are single-pass single-stage units. However, as discussed above, the ROmembrane units of the train may also be single-pass two-stage units. Theperson skilled in the art would understand how to modify the train ofFIG. 3 so that the RO membrane units are operated in single-passtwo-stage mode. The retentate from the RO membrane units of each row isrejected by being fed via a common retentate reject line (not shown) tothe RO retentate header (not shown). The NF retentate and RO retentateare optionally combined and are either discharged to the environment,for example, into the sea, or are injected down an injection well eitherinto a hydrocarbon bearing formation or an aquifer. The permeate fromthe RO membrane units of each row is fed via a common RO permeate line(not shown) to a permeate header (not shown) where it is combined withthe NF permeate. The NF permeate line is provided with a back pressurevalve to ensure that the pressure of the NF permeate is sufficientlyabove that of the RO permeate that the NF permeate can be metered intothe permeate header and mixed with the RO permeate thereby forming theinjection water stream.

Example 1

A low salinity injection water stream may be prepared from a sourcewater having a TDS content of 35,800 ppm, a sulfate anion concentrationof 2,750 ppm and a multivalent cation concentration (sum of the calciumand magnesium cation concentrations) of 1830 ppm by feeding the sourcewater at a rate of 320 thousand barrels of water per day (mbwd) to adesalination plant comprising a plurality of RO membrane units and aplurality of NF membrane units. The source water feed was divided toprovide an RO feed water for the RO membrane units (310 mbwd) and an NFfeed water for the NF membrane units (10 mbwd). The RO membrane unitswere operated at a pressure of 1000 psi absolute and a recovery of 50%by volume to provide 155 mbwd of an RO permeate stream having a TDS of177 ppm, a sulfate anion concentration of 1.5 ppm and a multivalentcation concentration of 2.5 ppm. The NF feed water for the NF membraneunits was reduced in pressure via a pressure let down valve to apressure of 400 psi absolute (the operating pressure of the NF membraneunits). The NF membrane units were operated at a recovery of 50% byvolume to provide 5 mbwd of an NF permeate stream having a totaldissolved solids content of 26,500 ppm, a sulfate concentration of 25ppm and a multivalent cation concentration of 132 ppm. The NF permeatestream and RO permeate stream were combined to give 160 mbwd of aninjection water stream having a TDS of 1000 ppm, a sulfate anionconcentration of 2.2 ppm and a multivalent cation concentration of 6.5ppm (using a blend ratio of RO permeate to NF permeate of 31.0:1).

Example 2

A low salinity injection water stream may be prepared from a sourcewater having a TDS content of 35,800 ppm, a sulfate anion concentrationof 2,750 ppm and a multivalent cation concentration (sum of the calciumand magnesium cation concentrations) of 1830 ppm by feeding the sourcewater at a rate of 320 thousand barrels of water per day (mbwd) to adesalination plant comprising a plurality of RO membrane units and aplurality of NF membrane units. The source water feed was divided toprovide an RO feed water for the RO membrane units (261.4 mbwd) and anNF feed water for the NF membrane units (58.6 mbwd). The RO membraneunits were operated at a pressure of 1000 psi absolute and a recovery of50% by volume to provide 130.7 mbwd of an RO permeate stream having aTDS of 177 ppm, a sulfate anion concentration of 1.5 ppm and amultivalent cation concentration of 2.5 ppm. The NF feed water for theNF membrane units was reduced in pressure via a pressure let down valveto a pressure of 400 psi absolute (the operating pressure of the NFmembrane units). The NF membrane units were operated at a recovery of50% by volume to provide 29.3 mbwd of an NF permeate stream having atotal dissolved solids content of 26,500 ppm, a sulfate concentration of25 ppm and a multivalent cation concentration of 132 ppm. The NFpermeate stream and RO permeate stream were combined to give 160 mbwd ofan injection water stream having a TDS of 5000 ppm, a sulfate anionconcentration of 5.8 ppm and a multivalent cation concentration of 26.2ppm (using a blend ratio of RO permeate to NF permeate of 4.5:1).

The invention claimed is:
 1. A process of producing an injection waterstream of controlled salinity and controlled sulfate anion content thatis suitable for injection into an oil bearing formation of an oilreservoir, the process comprising the steps of: (a) feeding a sourcewater having a total dissolved solids content in the range of 20,000 to45,000 ppm and a sulfate anion concentration in the range of 1,000 to4,000 ppm to a desalination plant that comprises a plurality of reverseosmosis (RO) membrane units and a plurality of nanofiltration (NF)membrane units wherein the source water is pressurised to a pressure inthe range of 350 to 1250 psi absolute by a pumping system common to saidplurality of RO membrane units and said plurality of NF membrane units,and dividing the source water to provide a feed water for the ROmembrane units (hereinafter “RO feed water”) and a feed water for the NFmembrane units (hereinafter “NF feed water”); (b) increasing thepressure of the RO feed water to a value in the range of 900 to 1250 psiabsolute if the RO feed water divided from the source water has apressure lower than 900 psi absolute before introducing the RO feedwater to the RO membrane units and withdrawing an RO permeate and an ROretentate from the RO membrane units wherein the RO membrane units areoperated in either a single-pass, single-stage mode or in a single-pass,two-stage mode and wherein the recovery of RO permeate is in the rangeof 35 to 75% by volume based on the volume of the RO feed water that isfed to the RO membrane units such that the RO permeate has a totaldissolved solids contents of less than 250 ppm, and a sulfate anionconcentration of less than 3 ppm; (c) reducing the pressure of the NFfeed water to a value in the range of 350 to 450 psi absolute if the NFfeed water divided from the source water has a pressure higher than 450psi absolute by means of a pressure let down valve or a control valvebefore introducing the NF feed water to the NF membrane units andwithdrawing an NF permeate and an NF retentate from the NF membraneunits wherein the NF membrane units are operated in a single-pass,single-stage mode and wherein the NF membrane units are operated with arecovery of NF permeate in the range of 35 to 60% by volume based on thevolume of the NF feed water that is fed to the NF membrane units suchthat the NF permeate has a total dissolved solids content in the rangeof 15,000 to 40,000 ppm and a sulfate anion concentration of less than40 ppm; and (d) mixing at least a portion of the RO permeate and atleast a portion of the NF permeate in a ratio in the range of 2:1 to40:1 at a mixing point, to provide an injection water having a totaldissolved solids content in the range of 500 to 5,000 ppm and a sulfateanion concentration of less than 7.5 ppm; and wherein the pressure ofthe NF permeate is maintained at least 5 psi higher than the pressure ofthe RO permeate by means of a back pressure valve that is providedupstream of the mixing point for the NF permeate and RO permeate andwherein the NF permeate is injected into the RO permeate at the mixingpoint to form the injection water.
 2. A process as claimed in claim 1wherein the source water has a multivalent cation concentration in therange of 700 to 3,000 ppm, the RO permeate has a multivalent cationcontent of up to 10 ppm, the NF permeate has a multivalent cationcontent of up to 200 ppm; and the injection water has a multivalentcation content of up to 50 ppm.
 3. A process as claimed in claim 1wherein energy is recovered from the RO retentate using a hydraulicrecovery unit and wherein the pressure of the RO feed water is increasedin step (b) using a booster pump that is coupled to the hydraulicrecovery unit.
 4. A process as claimed in claim 1 wherein the pressureof the NF feed water is reduced in pressure in step (c) to a pressure inthe range of 350 to 450 psi absolute by means of a pressure let downvalve.
 5. A process as claimed in claim 1 wherein the source water isselected from seawater, estuarine water, a produced water, an aquiferwater, and a waste water.
 6. A process as claimed in claim 1 wherein thetotal dissolved solids content of the RO permeate is in the range of 50to 225 ppm and the sulfate anion content of the RO permeate is at least0.5.
 7. A process as claimed in claim 1 wherein the total dissolvedsolids content of the NF permeate is not more than 15,000 ppm less thanthe total dissolved solids content of the source water and wherein theNF permeate has a sulfate anion concentration of at least 10 ppm.
 8. Aprocess as claimed in claim 2 wherein the concentration of multivalentcations in the RO permeate is in the range of 1 to 10 ppm and theconcentration of multivalent cations in the NF permeate is in the rangeof 50 to 200 ppm.
 9. A process as claimed in claim 1 wherein one or moreof the flow rate of the source water, the mixing ratio of the ROpermeate and NF permeate, and the flow rate of the injection water isdetermined in accordance with a measured variable that is selected fromone or more of the conductivity of the injection water, the totalconcentration of divalent anions in the injection water or in the NFpermeate, and the concentration of sulfate anions in the injection wateror in the NF permeate.
 10. A process as claimed in claim 4 wherein thepressure of the NF feed water is reduced in pressure in step (c) to apressure in the range of 380 to 420 psi absolute, by means of a pressurelet down valve.